1. Field of the Invention
The present invention relates to a data process, particularly to an encryption technique for data having a hierarchical structure.
2. Related Background Art
In recent years, various digital devices such as personal computers, PDAs, portable phones and the like have been developed and used. These digital devices have various functions such as screen size, resolution, color numbers, and the like with respect to image display. Various image formats are employed in order to cope with the multiplicity of display functions. Specifically, there is a type of format having images in a hierarchical manner like Flash Pix, JPEG2000. Some of these formats having a hierarchical structure hold only differential data relative to adjacent hierarchized image data as hierarchized image data, thereby reducing the amount of image data that has to be handled.
On the other hand, copyright protection of digital contents has been seen as a problem along with free utilization of the digital contents by high performance of digital devices and quality improvement of the digital contents.
With this situation as background, various inventions for realizing access control for each hierarchy have been made with respect to hierarchical images.
According to Japanese Patent Application Laid-Open No. 6-301754, an image is decomposed into a low resolution image and a high resolution image, and a key is used for only the high resolution image to perform encryption so that the low resolution image can be accessed without a key and the high resolution image can be accessed only by using the key.
Further, according to Japanese Patent No. 2606074, image data is decomposed into a plurality of spatial components at the transmission side, encoding is performed for each component, encryption is performed for each of the resulting sets of encoded data, and the encrypted data is multiplexed to be transmitted. At the reception side, the received multiplexed data is separated, each separated set of enciphered data is deciphered into unencrypted but encoded data, and each set of this encoded data is decoded into component data, and the image data is obtained by combining the several sets of component data.
In Japanese Patent Application Laid-Open No. 6-301754 described above, since the low resolution image data which is low hierarchical image data is not encrypted, access control for each level of the hierarchy of the image data cannot be realized. The keys used for encryption of the respective levels cannot be the same key, when it is considered that encryption is performed for each level. Further, when the hierarchized image data is differential data relative to adjacent hierarchized image data and access is made to specific hierarchized image data, not only the key corresponding to the hierarchized image data to be accessed but also the keys corresponding to the respective dependent hierarchized image data are also required.
Therefore, in order to realize access control for each level of the hierarchy of the hierarchized image data, a plurality of different keys are required. When making access to specific one-hierarchy image data, a plurality of keys may still be used, so that key management is complicated. However, Japanese Patent Application Laid-Open No. 6-301754 discloses only that an encrypted key is input by a user. Further, Japanese Patent No. 2606074 does not disclose the encrypted key management.
The upper limit of high resolution of an image has been increasingly high along with quality improvement of the image thanks to the increasingly high performance of digital devices in recent years. On the other hand, when utilization of images in a mobile device such as portable phone or PDA (Personal Digital Assistant) is considered, the lower limit of low resolution is not lowered, and does not become remarkably higher. This fact means that the width of the range of image resolutions used is increased, and that the number of hierarchies is increased in a hierarchical image. Therefore, when management is performed using a conventional method, the number of keys to be managed also is increased as the number of hierarchies is increased, so that the key management method is seen as a problem.
Moreover, there is a demand to reduce a problem relating to key distribution by utilizing a key possessed by a user when encrypted key management is considered.